Suppose the circumference of a circle is ${4\pi}$. What is its area?
Solution: ${c = 4\pi}$ ${K = 4\pi}$ First, find the radius: $r = c/2\pi = {4\pi}/2\pi = {2}$ Now find the area: $K = \pi r^2$, so $K = \pi \cdot {2}^2 = {4\pi}$.